Preventing the transmission of vibration and other movements from one body to another has been an important problem since the beginning of the machine age. The development of increasingly complex machines has resulted in the ubiquitous utilization in such machines of various approaches to vibration attenuation. Increases in the accuracy of tasks performed by various machines have demanded increasingly sophisticated and more tailored approaches to reducing the transmission of vibration. For several machine technologies, these approaches must not only account for internal vibrations that are transmitted from one portion of the machine to another, but also for external vibrations that may affect the work product of the machine.
An example of a machine technology in which demands on accuracy and precision are extreme is microlithography as used, for example, in the manufacture of microelectronic devices (e.g., integrated circuits). Microlithography involves the transfer of a pattern, used to define a layer of a microelectronic device, onto a sensitized surface of a suitable substrate such as a semiconductor wafer. Hence, microlithography is analogous to an extremely sophisticated photographic printing process. Modern microlithographic exposure apparatus (commonly called “steppers”) are capable of imprinting patterns in which the pattern elements, as imaged on the substrate, have linewidths at or about the wavelength of the light used to form the image. For example, certain modern steppers can form images of linear pattern elements having a linewidth of 0.25 or 0.18 μm, or even smaller, on the substrate. Achieving such a high level of performance requires that all imaging, positioning, and measuring systems of the stepper operate at their absolute limits of performance. This level of performance also requires that vibrations and other unwanted physical displacements be eliminated from the machine.
A conventional approach to vibration attenuation between two physical bodies involves the use of one or more air springs between the bodies. An air spring is a spring device in which the energy-storage element is air that is confined in a container that usually includes an elastomeric bellows or diaphragm. Air springs are commercially available in many different configurations and sizes and are used in a wide variety of applications with good success. A key attribute of an air spring is its reduced axial stiffness with respect to the load applied to the air spring. (Usually the load is applied axially relative to the air spring.) For many applications, especially in situations in which attenuation of axial motion is the objective, an air spring is sufficient for achieving satisfactory vibration attenuation.
A disadvantage of an air spring for certain applications is its relatively high lateral stiffness. The high lateral stiffness can result in significant transmission via the air spring of non-axial motions from one body to another. If the subject machine is one in which and/or from which substantially all vibrations must be isolated completely, an air spring will exhibit unsatisfactory performance. For example, in a stepper machine, any significant lateral stiffness in a vibration-attenuation device can cause problems with overlay accuracy of different layers as imaged on a wafer. Another possible problem in a stepper machine is an increased synchronization error between the reticle stage and the wafer stage.
Hence, in modern stepper machines and other types of vibration-sensitive equipment, there is a need for vibration attenuators that exhibit good vibration attenuation in the axial direction and that exhibit low or substantially zero lateral stiffness to prevent transmission of any vibrations between any of various portions of the machine.